The entry of calcium into cells through voltage-gated calcium channels mediates a wide variety of cellular and physiological responses, including excitation-contraction coupling, hormone secretion and gene expression (Miller, R. J., Science (1987) 235:46-52; Augustine, G. J. et al., Annu Rev Neurosci (1987) 10: 633-693). In neurons, calcium channels directly affect membrane potential and contribute to electrical properties such as excitability, repetitive firing patterns and pacemaker activity. Calcium entry further affects neuronal functions by directly regulating calcium-dependent ion channels and modulating the activity of calcium-dependent enzymes such as protein kinase C and calmodulin-dependent protein kinase II. An increase in calcium concentration at the presynaptic nerve terminal triggers the release of neurotransmitter and calcium channels, which also affects neurite outgrowth and growth cone migration in developing neurons.
Calcium channels mediate a variety of normal physiological functions, and are also implicated in a number of human disorders. Examples of calcium-mediated human disorders include but are not limited to congenital migraine, cerebellar ataxia, angina, epilepsy, hypertension, ischemia, and some arrhythmias. The clinical treatment of some of these disorders has been aided by the development of therapeutic calcium channel antagonists (e.g., dihydropyridines, phenylalkyl amines, and benzothiazapines all target L-type calcium channels) (Janis, R. J. & Triggle, D. J., In Calcium Channels: Their Properties, Functions, Regulation and Clinical Relevance (1991) CRC Press, London).
Native calcium channels have been classified by their electrophysiological and pharmacological properties into T-, L-, N-, P/Q- and R-types (reviewed in Catterall, W., Annu Rev Cell Dev Biol (2000) 16: 521-555; Huguenard, J. R., Annu Rev Physiol (1996) 58: 329-348). T-type (or low voltage-activated) channels describe a broad class of molecules that transiently activate at negative potentials and are highly sensitive to changes in resting potential.
The L-, N- and P/Q-type channels activate at more positive potentials (high voltage-activated) and display diverse kinetics and voltage-dependent properties (Catterall (2000) supra; Huguenard (1996) supra). L-type channels can be distinguished by their sensitivity to several classes of small organic molecules used therapeutically, including dihydropyridines (DHP's), phenylalkylamines and benzothiazepines. In contrast, N-type and P/Q-type channels are high affinity targets for certain peptide toxins produced by venous spiders and marine snails: N-type channels are blocked by the ω-conopeptides ω-conotoxin GVIA (ω-CTx-GVIA) isolated from Conus geographus and ω-conotoxin MVIIA (ω-CTx-MVIIA) isolated from Conus magus, while P/Q-type channels are resistant to ω-CTx-MVIIA but are sensitive to the funnel web spider peptide, ω-agatoxin IVA (ω-Aga-IVA). R-type calcium channels are sensitive to block by the tarantula toxin, SNX-482.
Neuronal high voltage-activated calcium channels are composed of a large (>200 kDa) pore-forming α1 subunit that is the target of identified pharmacological agents, a cytoplasmically localized ˜50-70 kDa β subunit that tightly binds the α1 subunit and modulates channel biophysical properties, and an ˜170 kDa α2δ subunit (reviewed by Stea, et al., Proc Natl Acad Sci USA (1994) 91:10576-10580; Catterall (2000) supra). At the molecular level, nine different α1 subunit genes expressed in the nervous system have been identified and shown to encode all of the major classes of native calcium currents.
Calcium channels have been shown to mediate the development and maintenance of the neuronal sensitization processes associated with neuropathic pain, and provide attractive targets for the development of analgesic drugs (reviewed in Vanegas, H. & Schaible, H-G., Pain (2000) 85: 9-18). All of the high-threshold Ca channel types are expressed in the spinal cord, and the contributions of L-, N and P/Q-types in acute nociception are currently being investigated. In contrast, examination of the functional roles of these channels in more chronic pain conditions strongly indicates a pathophysiological role for the N-type channel (reviewed in Vanegas & Schaible (2000) supra).
A considerable amount of effort has been undertaken by industry to develop an orally available N-type calcium channel antagonist. A series of potent N-type calcium channel antagonists have been disclosed in U.S. Pat. Nos. 6,294,533; 6,387,897; 6,951,862; and 6,949,554 and U.S. application Ser. Nos. 11/214,218 and 11/215,064.
While previous studies have demonstrated the ability of such compounds to block N-type calcium channels, there have not been any pharmacokinetic studies to evaluate the oral bioavailability of the compounds. There thus remains a need to better understand the pharmacokinetics and to correct any issues that may arise from such an understanding.
All patents, patent applications and publications disclosed herein are hereby incorporated by reference in their entirety.